The present invention relates generally to a suspension control for a vehicle and more particularly to suspension control method and apparatus for a vehicle suited for enhancing the comfortableness and manipulatability in driving the vehicle.
Heretofore, it is known to perform a suspension control for ensuring a manipulation stability of the vehicle (motor vehicle) such as an automobile by suppressing the rolling, pitching, bouncing and the like phenomena in the course of running as well as at the times of start and stop. In association with the suspension control, the wheels are connected to the body of car through interposed dampers (shock absorbers) with a view to improving the comfortableness to ride.
A suspension control for the motor vehicle is known in which the damper stroke positions are detected by means of damper stroke sensors to determine damping constants on the basis of the detected values for thereby controlling the variable dampers, as is disclosed in JP-A-61-143209 corresponding to U.S. pat. Ser. No. 681312 filed on Dec. 13, 1984 in the name of General Motors Co. According to this know suspension control, however, the damper control is carried out without discriminatively determining the sprung mass vibration (i.e. vibration of sprung mass) and the unsprung mass vibration (i.e. vibration of unsprung mass), as the result of which difficulty is encountered in assuring improved comfortableness to ride and good manipulatability in driving.
As an attempt to enhance the comfortableness in ride and the manipulatability in driving which are the conditions contradicting each other, there has been developed a so-called electronically controlled suspension system in which the overall control inclusive of controlling the spring constants of air springs and adjustment of height of car in addition to the suspension control can be electronically performed. This type suspension control system tends to be increasingly installed in the motor vehicles. A typical one of such electronic suspension control systems is disclosed, for example, in a Japanese periodical entitled "CAR AND MAINTENANCE", Vol. 39, No. 2, p.p. 41-52 (1985). According to this known control technique, road height sensors and acceleration sensors are mounted for performing the suspension control adaptively in consideration of vibrations in the road level or height and the side wind. Consequently, the control system is necessarily very expensive. Further, it must be pointed out that neither the bouncing nor the unsprung mass vibration can be discriminatively detected. Accordingly, with the suspension control in which no consideration is paid to the abovementioned phenomena, the improvement attainable in the comfortableness and the manipulatability is not to be sufficiently satisfactory.
Further, in the hitherto known suspension control, no consideration is paid to the damper stroke speed at the time of changing over the damper characteristics, giving rise to a problem that sprung mass is subjected to remarkable change in acceleration upon changing-over of the damper characteristics.